the input circuit is opamps anyway, which can be set up to have input impedance as high as 1M.
Higher impedance = higher noise, but 25K to 50K is more typical than 10K for a solid state amplifier. It’s not exactly low either to me... it is still 17x more than 600 Ohms.
I'm not surprised at all your preamps can drive 10K. ;-) Most should.